Wedged between a fitness center and a restaurant, the mosque called Masjid Taiba is
a small, unassuming building in the central quarter of Hamburg. In the late
1990s, the mosque's prayer rooms became a cauldron of terror  the place where
Mohammed Atta and his "Hamburg Cell" henchmen congregated before the deadly
9/11 attacks. On Monday, Aug. 9, German police raided the mosque, confiscated dozens
of computers and files and announced that because the spiritual home of
several of the 9/11 hijackers had once again become a magnet for violent
extremists, the mosque would be shut down.

"The Taiba mosque was the main meeting point for radical Islamic militants
in Hamburg and had been supporting the international terrorist network for
years," Manfred Murck, deputy director of Hamburg's intelligence service,
tells TIME. "Young Muslim men were drawn to the mosque  they met there,
prayed, chatted together, became radicalized and set up extremist groups."
Murck adds that the men came from different backgrounds, ranging from Moroccans and
German Islam converts to militants from the former Yugoslavia, Chechnya and the Middle East. They all had one thing in common  their commitment to jihad. "Some Muslims at the Taiba mosque had contacts
with al-Qaeda," claims Murck. (See pictures of the challenge of
memorializing 9/11.)

Intelligence officials say the mosque had been under continuous surveillance
since the 9/11 attacks. Back then, the mosque was known as al-Quds.
Hijackers Mohammed Atta, Marwan al-Shehhi, Ziad Jarrah and Ramzi
Binalshibh  who according to U.S. officials was a key 9/11 facilitator and
Atta's roommate  were all regulars at the prayer house in the run-up to the
2001 attacks.

In March 2009, the Taiba mosque again became a location of special interest
to investigators when a group of 11 Hamburg jihadists, mostly German men
with roots in the Middle East and the Caucasus, traveled to Afghanistan and
Pakistan to attend terrorist training camps. Officials say at least one man
who lived in Hamburg, identified by the intelligence services as Shahab D.,
joined the militant Islamic Movement of Uzbekistan (IBU), which is believed
to have close links with al-Qaeda. The Iranian-born man, in his mid-20s,
appeared in a terrorist video in October 2009 under the name of Abu Askar; he
urged other Muslims to join jihad and threatened attacks on Germany. Some
members of the group ended up in Pakistani custody and were sent back to
Germany, while other jihadists are still in Pakistani or U.S. custody. (See pictures of a jihadist's journey.)

Back at the Taiba mosque in Hamburg, intelligence officials say the men
were treated as "heroes," and as other Islamic militants gathered at the
mosque and tried to latch on to terrorist organizations in Afghanistan, a
form of "jihad tourism" developed.

In the raids on Aug. 9, the Taiba German-Arab cultural association that
ran the mosque was also banned by Hamburg's Interior Ministry, and the
group's assets were seized, although no arrests were made in any of the
day's raids. "We've closed the Taiba Mosque, as it was here that young men
were converted into religious fanatics," Christoph Ahlhaus, Hamburg's
Interior Minister, said in a statement. "Behind the scenes, an alleged
cultural organization has shamelessly exploited the freedoms of our
constitutional democracy to promote the cause of the 'holy war,' " he said,
pointing out that the courses, sermons and seminars at the mosque "spread an
ideology that was hostile to democracy" and sought to radicalize young
Muslims. Ahlhaus insisted that Hamburg "must not serve as the incubator for
Islamists willing to use violence."

The Taiba association could not be reached for comment about the police
raids, but German media outlets have reported that in the past, the
organization denied accusations of terrorist ties. According to the news
magazine Focus, the group wrote in October 2009 that it was the "victim of
a big media campaign, a bad secret-service plot and a concocted political
farce."

As part of the police operation, investigators in the city searched four apartments and
houses belonging to members of the Taiba association  again, no
arrests were made. German security sources tell TIME that one of the
apartments searched on Monday belonged to Mamoun
Darkazanli, a German-Syrian suspected al-Qaeda supporter who used to run an import-export
company in Hamburg and is wanted in Spain on terrorism charges. Darkazanli
was arrested in Hamburg in 2004 on a Spanish warrant but released in 2005
after a ruling by Germany's federal constitutional court. German
intelligence officials say he was one of the imams at the Taiba
mosque and have described him as a "hate preacher." "Darkazanli encouraged
young Muslim men to join the jihad with his emotional prayers," says Murck.
So far German prosecutors haven't been able to find evidence that he
supported al-Qaeda. (See more on Indonesia's arrest of the cleric linked to the Bali bombings.)

Now that the mosque has been closed, fears have been raised that members could
regroup and gather in other mosques in Hamburg. A large harbor city with a
thriving immigrant community, Hamburg has acted as a haven for Islamic
extremists in the past, as New York, Washington and Pennsylvania learned to
their horror nearly 10 years ago. A 2009 intelligence report from German
Domestic intelligence estimated that there are 2,000 Islamists in the city,
of whom 200 are bent on violence and 45 are jihadists with links to other
violent Islamic extremists in Germany and abroad. With one of their alleged
meeting points closed, the challenge for intelligence agencies now is
to prevent new terrorist cells from forming within their dispersed ranks.